In recent years, problems associated with aging of structures such as bridges built during the period of rapid economic growth become actualized. Damages of if accidents occur in the structures are incalculable. Therefore, technologies for monitoring states of the structures are known. For example, technologies for detecting damage of the structures by an acoustic emission (AE) system that detects elastic waves generated in association with generation of internal cracks or development of the internal cracks by high-sensitive sensors are known.
The acoustic emission is an elastic wave generated in association with development of fatigue cracks of material. In the AE system, this elastic wave is detected by an AE sensor using a piezoelectric element as a voltage signal (AE signal). The AE signal is detected as a premonition before rupture of the material is caused, and thus the frequency of generation of the AE signal and signal intensity of the AE signal are useful as indexes that indicate soundness of the material. Therefore, technologies for detecting an indication of deterioration of structures by the AE system have been actively studied. Especially, in corrosion diagnosis of oil tanks, manufacturing processes of mechanical devices, and the like, the detection technologies of the AE system have been widely used mainly in Europe and the United States, and standardization of the detection technologies of the AE system has been performed.
The AE signal is typically a feeble signal and a signal level thereof needs to be increased using an amplifier having high degree of amplification, and thus the AE signal is susceptible to noises while being highly sensitive. Therefore, in conventional technologies, the noises that are significant enough to cause erroneous determination of AE signal detection may be superimposed depending on an AE signal detection environment.